1) Field of the Invention
The present invention relates to a cleaning unit, a process cartridge equipped with the cleaning unit, an image forming apparatus, and a toner used in the image forming apparatus.
2) Description of the Related Art
Image forming process in electrophotography includes steps of charging a surface of a photosensitive drum, forming an electrostatic latent image on the surface of the photosensitive drum, developing the electrostatic latent image with toner to form a toner image, transferring the toner image to an intermediate transfer belt, transferring the toner image from the intermediate transfer belt to a recording medium or transferring the toner image on the photosensitive drum directly to the recording medium, and forming the image on the recording medium by fixing the toner with a hot roll. Some of the toner that is not transferred to the intermediate transfer belt or the recording medium remains on the surface of the photosensitive drum after having transferred the toner image. In addition, paper dust, additive component in the toner, etc. are deposited on the surface of the photosensitive drum. In General, a cleaning unit cleans all those foreign matter including the remained toner, etc. before proceeding to the next image forming process. The cleaning unit typically includes a blade, a brush, and a magnetic brush. Scraping the remaining toner off on the photosensitive drum by using a cleaning blade made of an elastic material is a generally accepted way of cleaning because of low cost and stable efficiency.
On the other hand, with an advancement of a digital technology, a digital image forming has gained a popularity. Particularly, in a color image forming apparatus, a highly defined image output by reproducing a small dot of one pixel is commonly demanded. To meet the demand for the highly defined image, reducing a particle size, improving a spherical shape of particles have been studied as a part of improvement in toner. Reproducibility of a dot is improved by reducing the particle size. Developing and transferring is enhanced by employing the spherical shape of particles.
However, if a toner having a small particle size with the spherical shape is used, the surface of the photosensitive drum cannot be cleaned properly with the cleaning blade after having transferred the toner image. This is because it is difficult to overcome adhesive power of toner on the photosensitive drum due to the small particle size. Moreover, due to the spherical particles in the toner, the toner tends to rotates between the cleaning blade and the photosensitive drum thereby slipping through the gap.
If a pressure under which the cleaning blade is in contact with the photosensitive drum is increased to clean the toner having a small particle size with the spherical shape, with increased force of friction, it results in bending or chattering of the blade, thereby affecting the cleaning of the photosensitive drum.
The temperature dependency is one of the mechanical properties of the cleaning blade. In a wide range of temperatures under which the image forming apparatus is used, particularly, at low temperatures, the scraping capacity of the cleaning blade with respect to the photosensitive drum is deteriorated, thereby causing slipping of the toner on the surface of the photosensitive drum.
The following measures are proposed for cleaning the photosensitive drum when the toner having a small particle size with the spherical shape is used.
In the image forming apparatus disclosed in Japanese Patent Application Laid Open Publication No. 2002-268490, a polymer toner has an average particle size in a range of 3 micrometers to 8 micrometers, a variation coefficient in a particle size distribution not more than 27 percent, and proportion of toner particles in a range of shape factor from 0.940 to 0.985. Material for cleaning blade in the cleaning unit is polyurethane rubber having JIS rubber hardness in a range of 65 to 73 degrees in an environment of 25° C., 300 percent modulus 980×104 Pascal, an impact resilience in a range of 40 percent to 73 percent, and the cleaning blade in contact is exerting load in a range of 147 mN/cm to 147 mN/cm on the photosensitive drum.
Moreover, in the cleaning unit disclosed in Japanese Patent Application Laid Open Publication No. 2002-214992, a cleaning blade having the impact resilience H fulfilling the condition 45≦H<70 (percent) is allowed to be in contact with a cylindrical photosensitive drum at an angle not more than central angle of cylinder β±30 degrees.
Furthermore, in the image forming apparatus disclosed in Japanese Patent Application Laid Open Publication No. 2002-72804, a toner prepared by adding an external additive having an average particle size in a range of 50 micrometers to 500 micrometers to toner particles having an average shape index SF in a range of 100 to 135 is used. A cleaning blade of the cleaning unit is made of an elastic material having an impact resilience not les than 35 percent at 10° C.
Moreover, an additive like fine particles of resin or a metal oxide, a metal nitride, and metal carbide of silica, titania, alumina etc. are added to the toner to adjust fluidity and charging characteristics of the toner. However, if the particles are made smaller, the surface area of a unit weight of toner becomes large and the quantity of the additive increases in accordance with the surface area. Due to the increase in the quantity of the additive to be added, the quantity of the additive that is separated (free) increases and the additive remains on an image carrier instead of being transferred together with the toner or being cleaned from the image carrier. Since the remained additive is insulating in nature electrically, it is charged by a charging unit and adheres firmly on the photosensitive drum. With this additive of silica etc. adhered as a base, the other ingredients of toner like a binder resin, a mold releasing agent etc. are adhered and deposited. This deposition of silica etc. on the photosensitive drum causes a faulty image with a white patch on a half tone or a beta image in negative-positive developing in digital developing.
In Japanese Patent Application Laid Open Publication No. 2000-19918 an image forming apparatus provided with two cleaning units where a peak temperature of tan δ of a cleaning blade in the second cleaning unit is adjusted in a range of 2° C. to 15° C. to deal with improper cleaning caused by almost spherical-shaped particles of toner due to polymerization process, is proposed. In Japanese Patent Application Laid Open Publication No. 2000-112315, a rubber member used in electrophotography made of an elastic rubber having a peak temperature of tan δ in a range of −10° C. to 20° C. and half power bandwidth not less than 30° C. is proposed. Moreover, in Japanese Patent Application Laid Open Publication No. 2001-290404, a cleaning blade made of urethane having a peak temperature of tan δ not less than 10° C. and not more than 30° C. is proposed.
Although the image forming apparatuses are used in wide range of environmental conditions from high temperature to low temperature, due to the temperature dependency, which is one of the mechanical properties of the cleaning blade, the cleaning blade resonates at a high temperature causing noise and chatters at a low temperature causing vibrations. Moreover, at a low temperature, due to deterioration of scraping of the photosensitive drum, the toner slips on the surface of the photosensitive drum. While using a toner having a small particle size with the spherical shape, the slipping of toner doesn't allow proper cleaning of the surface of the photosensitive drum. If the frictional force between the photosensitive drum and the cleaning blade is increased to achieve better cleaning, it results in bending or chattering of the blade, thereby affecting the cleaning of the photosensitive drum. Furthermore, in a toner having a small particle size, the increased amount of the external additive gives rise to image defects initiated from silica etc.